Citrulline Subsided Nickel Toxicity in Rice (Oryza sativa L.) by Modulating Photosynthesis, Metabolic Dysregulations, Metal Sequestration and Nutrient Uptake

Abstract: This study appraised the potential of citrulline (CTL) seed priming in mitigating nickel (Ni) toxicity in rice cultivars. Plants were subjected to Ni toxicity (80 and 100 mg kg^–1) with or without CTL seed priming (1, 1.5, and 2 mM). The results revealed that Ni-exposed plants exhibited substantial Ni uptake and accumulation in roots and leaves, leading to severe phytotoxicity. Excess Ni triggered oxidative stress by increasing the production of superoxide radicals, hydroxyl radicals, and hydrogen peroxide, alongside a surge in oxidized glutathione levels and lipoxygenase activity. This oxidative imbalance significantly compromised membrane integrity, as indicated by excessive malondialdehyde accumulation and heightened electrolyte leakage. Nickel toxicity augmented methylglyoxal (MG) production, aggravating cellular damage. Plants under Ni toxicity displayed drop in leaf relative water content and nutrient uptake. Nickel toxicity effects were more pronounced in the Ni-sensitive cultivar (PK-1121) than in the Ni-tolerant cultivar (Basmati-515). However, CTL lessened Ni accumulation in rice cultivars under Ni toxicity. Further, CTL improved activities of superoxide dismutase, peroxidase, catalase, ascorbate peroxidase, and glutathione S-transferase alongside levels of ascorbate, antioxidant pigments, which diminished oxidative stress. Citrulline enhanced endogenous reduced glutathione pool that facilitated MG detoxification in Ni-stressed plants. Citrulline increased osmolyte accumulation, which resulted in lower membrane damage and better water and nutrient acquisition. Higher endogenous hydrogen sulfide and nitric oxide levels in CTL-treated plants further stimulated the antioxidant system of stressed plants. These results suggest that CTL application can be a potential strategy to alleviate the adverse effects of Ni in rice plants.